DCL30-C. Declare objects with appropriate storage durations

An object has a storage duration that determines its lifetime. There are three storage durations: static, automatic, and allocated.

According to C99 [[ISO/IEC 9899-1999]]:

The lifetime of an object is the portion of program execution during which storage is guaranteed to be reserved for it. An object exists, has a constant address, and retains its last-stored value throughout its lifetime. If an object is referred to outside of its lifetime, the behavior is undefined. The value of a pointer becomes indeterminate when the object it points to reaches the end of its lifetime.

Attempting to access an object outside of its lifetime could result in an exploitable vulnerability.

Non-Compliant Code Example (Global Variables)

This non-compliant code example declares the variable p as a pointer to a constant char with file scope. The value of str is assigned to p within the dont_do_this() function. However, str has automatic storage duration, so the lifetime of str ends when the dont_do_this() function exits.

const char *p;
void dont_do_this() {
    const char str[] = "This will change";
    p = str; /* dangerous */
    /* ... */
}

void innocuous() {
    const char str[] = "Surprise, surprise";
}
/* ... */
dont_do_this();
innocuous();
/* now, it is likely that p is pointing to "Surprise, surprise" */

As a result of this undefined behavior, it is likely that p will refer to the string literal "Surprise, surprise" after the call to the innocuous() function.

Compliant Solution (p with Block Scope)

In this compliant solution, p is declared with the same scope as str, preventing p from taking on an indeterminate value outside of this_is_OK().

void this_is_OK() {
    const char str[] = "Everything OK";
    const char *p = str;
    /* ... */
}
/* pointer p is now inaccessible outside the scope of string str */

Compliant Solution (p with File Scope)

If it is necessary for p to be defined with file scope, it can be set to NULL before str is destroyed. This prevents p from taking on an indeterminate value, although any references to p must check for NULL.

const char *p;
void is_this_OK() {
    const char str[] = "Everything OK?";
    p = str;
    /* ... */
    p = NULL;
}

Non-Compliant Code Example (Return Values)

In this example, the function init_array() incorrectly returns a pointer to a local stack variable.

char *init_array() {
   char array[10];
   /* Initialize array */
   return array;
}

Some compilers generate a warning when a local stack variable is returned from a function. Compile your code at high warning levels and resolve any warnings (see MSC00-A. Compile cleanly at high warning levels).

Compliant Solution (Return Values)

Correcting this example depends on the intent of the programmer. If the intent is to modify the value of array and have that modification persist outside of the scope of init_array(), the desired behavior can be achieved by declaring array elsewhere and passing it as an argument to init_array().

int main(int argc, char *argv[]) {
   char array[10];
   init_array(array);
   /* ... */
   return 0;
}


void init_array(char array[]) {
   /* Initialize array */
   return;
}

Risk Assessment

Referencing an object outside of its lifetime could result in an attacker being able to run arbitrary code.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Automated Detection

Coverity Prevent. The RETURN_LOCAL checker finds many instances where a function will return a pointer to a local stack variable. Coverity Prevent cannot discover all violations of this rule, so further verification is necessary.

References

[[Coverity 07]]
[[ISO/IEC 9899-1999]] Section 6.2.4, "Storage durations of objects," and Section 7.20.3, "Memory management functions"

DCL12-A. Create and use abstract data types      02. Declarations and Initialization (DCL)       DCL32-C. Guarantee identifiers are unique